Chapter 13

How Populations Evolve

Biology and Society: Persistent Pests• Mosquitoes and malaria

– In the 1960s, the World Health Organization (WHO) began a campaign to eradicate the mosquitoes that transmit malaria.

– It used DDT, to which some mosquitoes have evolved resistance.

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• The evolution of pesticide-resistant insects is just one of the ways that evolution affects our lives.

• An understanding of evolution informs every field of biology, for example:

– Agriculture

– Medicine

– Biotechnology

– Conservation biology

CHARLES DARWIN AND THE ORIGIN OF SPECIES• Charles Darwin published On the Origin of Species by Means of

Natural Selection, November 24, 1859.

• Darwin presented two main concepts:– Life evolves

– Change occurs as a result of “descent with modification,” with natural selection as the mechanism

• Natural selection is a process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are individuals with other characteristics.

A Trinidad tree mantid that mimics dead leaves

Figure 13.1a

A leaf mantid in Costa RicaFigure 13.1b

A flower mantid in Malaysia Figure 13.1c

• Natural selection leads to:– A population (a group of individuals of the same species living in the

same place at the same time) changing over generations

– Evolutionary adaptation

• In one modern definition of evolution, the genetic composition of a population changes over time.

1809Lamarck

publisheshis theory

of evolution.

1830Lyell publishes

Principles of Geology.

1837Darwin begins analyzing his

specimens and writing hisnotebooks on the origin

of species. 1844Darwin writes his essayon the origin of species. 1865

Mendel publishespapers on genetics.

1858Wallace sends an

account of histheory to Darwin.

1859Darwin publishesThe Origin of Species.

1809Charles Darwinis born.

1831–36Darwin travels

around the worldon the HMS Beagle.

Green sea turtle in theGalápagos Islands

1800

1870

Figure 13.2

Green sea turtle in the Galápagos IslandsFigure 13.2a Figure 13.2b

Figure 13.2c Figure 13.2d

Darwin’s Cultural and Scientific Context• The Origin of Species challenged the notion that the Earth was:

– Relatively young

– Populated by unrelated species

The Idea of Fixed Species• The Greek philosopher Aristotle held the belief that species are

fixed and do not evolve.

• The Judeo-Christian culture fortified this idea with a literal interpretation of the Bible and suggested the Earth may only be 6,000 years old.

Lamarck and Evolutionary Adaptations• In the mid-1700s, the study of fossils began to take form as a

branch of science.

• Naturalist Georges Buffon noted that:– The Earth may be more than 6,000 years old

– There are similarities between fossils and living species

– Fossil forms might be ancient versions of similar living species

• Jean Baptiste Lamarck suggested that organisms evolved by the process of adaptation by the inheritance of acquired characteristics, now known to be incorrect.

The Voyage of the Beagle• Darwin was born on February 12, 1809, the same day that

Abraham Lincoln was born.

• In December 1831 Darwin left Great Britain on the HMS Beagle on a five-year voyage around the world.

Darwin in 1840Figure 13.3a

HMS BeagleFigure 13.3b

Galápagos Islands

PACIFICOCEANPinta

40 miles

40 km

0 Florenza

0

Fernandina

MarchenaGenovesa

Equator

Santiago

Daphne IslandsPinzón

Española

Isabela SantaCruz Santa

Fe SanCristobal

Figure 13.3c

• On his journey on the Beagle, Darwin:– Collected thousands of specimens

– Observed various adaptations in organisms

• Darwin was intrigued by:– The geographic distribution of organisms on the Galápagos Islands

– Similarities between organisms in the Galápagos and those in South America

Figure 13.4a Figure 13.4b

• Darwin was strongly influenced by the writings of geologist Charles Lyell.

• Lyell suggested that the Earth:– Is very old

– Was sculpted by gradual geological processes that continue today

• Darwin applied Lyell’s principle of gradualism to the evolution of life on Earth.

Descent with Modification• Darwin made two main points in The Origin of Species:

– Organisms inhabiting Earth today descended from ancestral species

– Natural selection was the mechanism for descent with modification

EVIDENCE OF EVOLUTION• Biological evolution leaves observable signs.

• We will examine five of the many lines of evidence in support of evolution:

– The fossil record

– Biogeography

– Comparative anatomy

– Comparative embryology

– Molecular biology

The Fossil Record• Fossils are:

– Imprints or remains of organisms that lived in the past

– Often found in sedimentary rocks

• The fossil record:– Is the ordered sequence of fossils as they appear in rock layers

– Reveals the appearance of organisms in a historical sequence

– Fits the molecular and cellular evidence that prokaryotes are the ancestors of all life

Figure 13.5

• Paleontologists:– Are scientists that study fossils

– Have discovered many transitional forms that link past and present

Figure 13.6-3

Biogeography• Biogeography is the study of the geographic distribution of

species that first suggested to Darwin that today’s organisms evolved from ancestral forms.

• Many examples from biogeography would be difficult to understand, except from an evolutionary perspective.

• One example is the distribution of marsupial mammals in Australia.

Commonringtailpossum

Red kangarooCommon wombat

Australia

Koala

Figure 13.7

Comparative Anatomy• Comparative anatomy

– Is the comparison of body structure between different species

– Confirms that evolution is a remodeling process

• Homology is:– The similarity in structures due to common ancestry

– Illustrated by the remodeling of the pattern of bones forming the forelimbs of mammals

Human Cat Whale Bat

Figure 13.8

• Vestigial structures:– Are remnants of features that served important functions in an organism’s

ancestors

– Now have only marginal, if any, importance

Comparative Embryology• Early stages of development in different animal species reveal

additional homologous relationships.– For example, pharyngeal pouches appear on the side of the embryo’s

throat, which:

– Develop into gill structures in fish

– Form parts of the ear and throat in humans

– Comparative embryology of vertebrates supports evolutionary theory.

Post-analtail

Human embryoChicken embryo

Pharyngealpouches

Figure 13.9

Molecular Biology• The hereditary background of an organism is documented in:

– Its DNA

– The proteins encoded by the DNA

• Evolutionary relationships among species can be determined by comparing:

– Genes

– Proteins of different organisms

Percent of selected DNA sequencesthat match a chimpanzee’s DNA

Chimpanzee

100%96%92%

Human

Gibbon

Orangutan

Gorilla

Primate

Old Worldmonkey

Figure 13.10

NATURAL SELECTION• Darwin noted the close relationship between adaptation to the

environment and the origin of new species.

• The evolution of finches on the Galápagos Islands is an excellent example.

(a) The largeground finch

(b) The small tree finch (c) The woodpecker finchFigure 13.11

Darwin’s Theory of Natural Selection• Darwin based his theory of natural selection on two key

observations:– All species tend to produce excessive numbers of offspring

– Organisms vary, and much of this variation is heritable

• Observation 1: Overproduction– All species tend to produce excessive numbers.

– This leads to a struggle for existence.

Sporecloud

Figure 13.12

• Observation 2: Individual variation– Variation exists among individuals in a population.

– Much of this variation is heritable.

Figure 13.13

• Inference: Differential reproductive success(natural selection)

– Those individuals with traits best suited to the local environment generally leave a larger share of surviving, fertile offspring.

Natural Selection in Action• Examples of natural selection include:

– Pesticide-resistant insects

– Antibiotic-resistant bacteria

– Drug-resistant strains of HIV

Chromosome with geneconferring resistanceto pesticide

Reproduction

Survivors

Insecticide application

Figure 13.14-3

• Observation: Flat-tailed horned lizards defend against attack by:– Thrusting their heads backward

– Stabbing a shrike with the spiked horns on the rear of their skull

• Question: Are longer horns a survival advantage?

• Hypothesis: Longer horns are a survival advantage.

The Process of Science: Does Predation Drive the Evolution of Lizard Horn Length?

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Side horns(tip to tip)

Killed Live

KilledLive

Rear horns0

10

20

Leng

th (m

m)

(c) Results of measurement of lizard horns(b) The remains of a lizard impaledby a shrike

(a) A flat-tailed horned lizard

Figure 13.15

• Prediction: Live horned lizards have longer horn lengths than dead ones.

• Experiment: Measure the horn lengths of dead and living lizards.

• Results: The average horn length of live lizards is about 10% longer than that of dead lizards.

Side horns(tip to tip)

Killed Live

KilledLive

Rear horns0

10

20

Leng

th (m

m)

(c) Results of measurement of lizard hornsFigure 13.15c

EVOLUTIONARY TREES• Darwin saw the history of life as analogous to a tree:

– The first forms of life on Earth form the common trunk

– At each fork is the last common ancestor to all the branches extending from that fork

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Tetrapodlimbs

Amnion

Feathers

Lungfishes

Mammals

Amphibians

Lizardsand snakes

Crocodiles

Hawks and other birds

Ostriches

Am

niotesTetrapods

Birds

Figure 13.16

The Modern Synthesis: Darwinism Meets Genetics• The modern synthesis is the fusion of genetics with evolutionary

biology.

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Populations as the Units of Evolution• A population is:

– A group of individuals of the same species, living in the same place, at the same time

– The smallest biological unit that can evolve

(a) Two dense populations oftrees separated by a lake

Figure 13.17a

(b) A nighttime satellite view of North America Figure 13.17b

• The total collection of alleles in a population at any one time is the gene pool.

• When the relative frequency of alleles changes over a number of generations, evolution is occurring on its smallest scale, which is sometimes called microevolution.

Genetic Variation in Populations• Individual variation abounds in populations.

– Not all variation in a population is heritable.

– Only the genetic component of variation is relevant to natural selection.

• Variable traits in a population may be:– Polygenic, resulting from the combined effects of several genes or

– Determined by a single gene

• Polygenic traits tend to produce phenotypes that vary more or less continuously.

• Single gene traits tend to produce only a few distinct phenotypes.

Figure 13.18

Sources of Genetic Variation• Genetic variation results from:

– Mutations, changes in the DNA of an organism

– Sexual recombination, the shuffling of alleles during meiosis

• For any one gene, mutation alone has little effect on a large population in a single generation.

• Organisms with very short generation spans, such as bacteria, can evolve rapidly with mutations as the only source of genetic variation.

Analyzing Gene Pools• The gene pool is a reservoir from which the next generation draws

its genes.

• Alleles in a gene pool occur in certain frequencies.

• Alleles can be symbolized by:– p for the relative frequency of the dominant allele in the population

– q for the frequency of the recessive allele in the population

• Genotype frequencies:– Can be calculated from allele frequencies

– Are symbolized by the expressions p2, 2pq, and q2

Figure 13.19

Allele frequencies

Genotype frequencies

Sperm

Eggs

p = 0.8(R)

q = 0.2(r)

p = 0.8

R

q = 0.2

r

RR

p = 0.8R

q = 0.2

r

p2 = 0.64

rR qp = 0.16 q2 = 0.04

rr

pq = 0.16 Rr

(RR) p2 = 0.64 q2 = 0.04

(rr)

2pq = 0.32 (Rr)

Figure 13.20

• The Hardy-Weinberg formula can be used to calculate the frequencies of genotypes in a gene pool from the frequencies of alleles.

Population Genetics and Health Science• The Hardy-Weinberg formula can be used to calculate the

percentage of a human population that carries the allele for a particular inherited disease.

• PKU:– Is a recessive allele that prevents the breakdown of the amino acid

phenylalanine

– Occurs in about one out of every 10,000 babies born in the United States

INGREDIENTS: SORBITOL,MAGNESIUM STEARATE,ARTIFICIAL FLAVOR,ASPARTAME† (SWEETENER),ARTIFICIAL COLOR(YELLOW 5 LAKE, BLUE 1LAKE), ZINC GLUCONATE.†PHENYLKETONURICS:CONTAINS PHENYLALANINE

Figure 13.21

Microevolution as Change in a Gene Pool• How can we tell if a population is evolving?

• A non-evolving population is in genetic equilibrium, called the Hardy-Weinberg equilibrium, in which the population gene pool remains constant over time.

• From a genetic perspective evolution can be defined as a generation-to-generation change in a population’s frequencies of alleles, sometimes called microevolution.

MECHANISMS OF EVOLUTION• The main causes of evolutionary change are:

– Genetic drift

– Gene flow

– Natural selection

Genetic Drift• Genetic drift is:

– A change in the gene pool of a small population

– Due to chance

Only 5 of10 plantsleaveoffspring

RR

rr

Rr

RR

RR

RR

RR Rr

RR

Rr

Rr

Only 2 of10 plantsleaveoffspring

RR

rr RR

Rr

rr

RR

Rr

Rr Rr

rr

RR

RR

RR

RR

RR

RR

RR

RR RR

Generation 1p (frequency of R) = 0.7

q (frequency of r) = 0.3

Generation 2 p = 0.5

q = 0.5

Generation 3 p = 1.0

q = 0.0

Figure 13.22-3

The Bottleneck Effect• The bottleneck effect:

– Is an example of genetic drift

– Results from a drastic reduction in population size

Originalpopulation

Bottleneckingevent

Survivingpopulation

Figure 13.23-3

• Bottlenecking in a population usually reduces genetic variation because at least some alleles are likely to be lost from the gene pool.

• Cheetahs appear to have experienced at least two genetic bottlenecks in the past 10,000 years.

Figure 13.24

The Founder Effect• The founder effect is likely when a few individuals colonize an

isolated habitat and represent genetic drift in a new colony.

• The founder effect explains the relatively high frequency of certain inherited disorders among some small human populations.

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SouthAmerica

Tristan da Cunha

Africa

Figure 13.25

Gene Flow• Gene flow:

– Is genetic exchange with another population

– Tends to reduce genetic differences between populations

Figure 13.26

Natural Selection: A Closer Look• Of all causes of microevolution, only natural selection promotes

adaptation.

Darwinian Fitness• Fitness is the contribution an individual makes to the gene

pool of the next generation relative to the contributions of other individuals.

Figure 13.27

Three General Outcomes of Natural Selection• Directional selection:

– Shifts the phenotypic “curve” of a population

– Selects in favor of some extreme phenotype

• Disruptive selection can lead to a balance between two or more contrasting phenotypic forms in a population.

• Stabilizing selection:– Favors intermediate phenotypes

– Is the most common

Originalpopulation

Evolvedpopulation Phenotypes (fur color)

Freq

uenc

yof

in

divi

dual

s

Originalpopulation

(a) Directional selection (b) Disruptive selection (c) Stabilizing selectionFigure 13.28

Sexual Selection• Sexual dimorphism is:

– A distinction in appearance between males and females

– Not directly associated with reproduction or survival

• Sexual selection is a form of natural selection in which inherited characteristics determine mating preferences.

(a) Sexual dimorphism in a finch speciesFigure 13.29a

(b) Competing for matesFigure 13.29b

Evolution Connection: The Genetics of the Sickle-Cell Allele• Sickle-cell disease:

– Is a genetic disorder

– Affects about one out of every 400 African-Americans

• Abnormally shaped red blood cells cause painful and life-threatening complications.

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• Heterozygous individuals for the sickle-cell allele:– Do not develop sickle-cell anemia

– Are more resistant to malaria

• In the African tropics, where malaria is most common, the frequency of the sickle-cell allele is highest.

Areas with highincidence ofmalaria

Frequencies of thesickle-cell allele

0–2.5%

10.0–12.5%

2.5–5.0%

5.0–7.5%

7.5–10.0%

•12.5%Figure 13.30a

Col

oriz

ed S

EM

Figure 13.30b